Niobium oxide is exemplary of the metal oxides which can be produced according to the present invention and is an important intermediate for the production of pure niobium metal and high purity, i.e., vacuum grade, ferroniobium. It is, therefore, desirable to obtain substantially pure niobium oxide for the subsequent reduction reaction to niobium metal or by metallothermic reaction with iron to form vacuum grade ferroniobium.
A ready source of niobium is available in the form of commercial grade ferroniobium alloys containing 20% to 40% by weight iron (Fe); and including minor amounts of tantalum, phosphorous, titanium and silicon. The presently available processes for the recovery of niobium oxide (Nn.sub.2 O.sub.5) from this source of FeNb include the chlorination of FeNb followed by a high temperature separation of the vapor phases of ferric chloride (FeCl.sub.3) from the niobium pentachloride (NbCl.sub.5) produced by passing the vapors of those chlorides through a bed of sodium chloride (NaCl) where the FeCl.sub.3 forms a eutectic composition with the NaCl, removing it from the vapor stream. Niobium chloride is then recovered by cooling the salt vapor to condense the (NbCl.sub.5). The NbCl.sub.5 can then optionally be distilled if desired before hydrolysis. Conventionally, NbCl is hydrolyzed by its addition to water which can be subsequently neutralized, and dried, before calcining in a heated kiln to produce relatively pure Nb.sub.2 O.sub.5. The drying and calcining is both energy intensive and expensive.
The preparation of Nb.sub.2 O.sub.5 by the described chlorination route utilizes toxic chlorine gas reacted exothermically at elevated temperatures and pressures. These conditions produce severe corrosion problems. Special equipment is necessary for handling the highly pressurized, corrosive liquid chlorine and it must be safely vaporized, metered and fed into the reactor. Likewise, the most suitable material for reactor construction is graphite. This is a brittle material which can fracture and fail abruptly after a short time in use in this environment. Further, the chlorine is normally used in excess to ensure complete reaction with the FeNb and the excess must be neutralized creating an expensive, undesirable by-product.
In addition to the foregoing, the hydrolysis step involves contacting the condensed chloride product with a neutralizing agent such as ammonia, and then filtering the resulting hydrous oxide slurry or cake, then firing it to oxide in a kiln. Such slurries and cakes are gelatinous and therefore hard to handle. There is need for a process which doesn't have to deal with such intractable intermediate products.